This invention relates generally to cryogenic air separation and, more particularly, to cryogenic air separation employing a reversing heat exchanger to clean and cool feed air prior to its passing into the cryogenic air separation plant.
In the practice of cryogenic air separation to produce one or more products such as nitrogen and oxygen, the feed air to the cryogenic air separation plant must be cleaned of high boiling impurities, such as carbon dioxide and water vapor, before it enters the cryogenic air separation plant because such high boiling impurities will freeze within the plant at the very low temperatures at which the plant operates, thus reducing the operating efficiency of the cryogenic air separation plant.
One very important system for cleaning the feed air is by the use of a reversing heat exchanger wherein the high boiling impurities freeze out onto the heat exchanger passages as the feed air is cooled against return streams such as product and waste streams, and periodically the flow of the feed air stream and a waste stream are alternated in the heat exchanger passages so that the deposited high boiling impurities are passed out of the heat exchanger with the waste stream.
Another important function in the operation of a cryogenic air separation system is the desuperheating of the compressed feed air before it enters the column(s) of the cryogenic air separation plant.
The heat exchangers employed to cool, clean and desuperheat the feed air for a cryogenic air separation plant involve considerable capital costs. Any improvement to such heat exchanger arrangement would be highly desirable.
Accordingly, it is an object of this to provide an improved heat exchanger system for treating feed air to a cryogenic air separation plant.
The above and other objects, which will become apparent to those skilled in the art upon a reading of this disclosure, are attained by the present invention, one aspect of which is:
A cryogenic air separation method comprising:
(A) cooling feed air by upward flow in a main heat exchanger to produce cooled feed air, and dividing the cooled feed air into a first portion and a second portion;
(B) partially condensing the first portion of the cooled feed air by downward flow in the main heat exchanger, and passing the partially condensed first portion of the cooled feed air and the second portion of the cooled feed air into a cryogenic air separation plant;
(C) separating the feed air by cryogenic rectification in the cryogenic air separation plant to produce at least one product; and
(D) warming said at least one product by downward flow in the main heat exchanger, and recovering said at least one product.
Another aspect of the invention is:
Cryogenic air separation apparatus comprising:
(A) a main heat exchanger and a cryogenic air separation plant comprising at least one column;
(B) means for passing feed air upwardly through a first section of the main heat exchanger, and means for passing feed air downwardly through a second section of the main heat exchanger;
(C) means for passing feed air from the main heat exchanger to the cryogenic air separation plant; and
(D) means for passing product from the cryogenic air separation plant to the main heat exchanger, and means for recovering product from the main heat exchanger.
As used herein, the term xe2x80x9cfeed airxe2x80x9d means a mixture comprising primarily nitrogen and oxygen, such as ambient air.
As used herein, the terms xe2x80x9cturboexpansionxe2x80x9d and xe2x80x9cturboexpanderxe2x80x9d mean respectively method and apparatus for the flow of high pressure fluid through a turbine to reduce the pressure and the temperature of the fluid thereby generating refrigeration.
As used herein, the term xe2x80x9ccolumnxe2x80x9d means a distillation or fractionation column or zone, i.e. a contacting column or zone wherein liquid and vapor phases are countercurrently contacted to effect separation of a fluid mixture, as for example, by contacting or the vapor and liquid phases on a series of vertically spaced trays or plates mounted within the column and/or on packing elements which may be structured packing and/or random packing elements. For a further discussion of distillation columns, see the Chemical Engineers"" Handbook fifth edition, edited by R. H. Perry and C. H. Chilton, McGraw-Hill Book Company, New York, Section 13, The Continuous Distillation Process.
Vapor and liquid contacting separation processes depend on the difference in vapor pressures for the components. The high vapor pressure (or more volatile or low boiling) component will tend to concentrate in the vapor phase whereas the low vapor pressure (or less volatile or high boiling) component will tend to concentrate in the liquid phase. Partial condensation is the separation process whereby cooling of a vapor mixture can be used to concentrate the more volatile component(s) in the vapor phase and thereby the less volatile component(s) in the liquid phase. Rectification, or continuous distillation, is the separation process that combines successive partial vaporizations and condensations as obtained by a countercurrent treatment of the vapor and liquid phases. The countercurrent contacting of she vapor and liquid phases is adiabatic and can include integral or differential contact between the phases. Separation process arrangements that utilize the principles of rectification to separate mixtures are often interchangeably termed rectification columns, distillation columns, or fractionation columns. Cryogenic rectification is a rectification process carried out, at least in part, at temperatures at or below 150 degrees Kelvin (K).
As used herein, the term xe2x80x9cindirect heat exchangexe2x80x9d means the bringing of two fluids into heat exchange relation without any physical contact or intermixing of the fluids with each other.
As used herein, the term xe2x80x9ccryogenic air separation plantxe2x80x9d means the column or columns wherein feed air is separated by cryogenic rectification, as well as interconnecting piping, valves, heat exchangers and the like.
As used herein, the terms xe2x80x9cupper portionxe2x80x9d and xe2x80x9clower portionxe2x80x9d of a column means those portions respectively above and below the midpoint of the column.
As used herein, the term xe2x80x9ctop condenserxe2x80x9d means a heat exchange device that generates column downflow liquid from column vapor.
As used herein, the term xe2x80x9cproduct nitrogenxe2x80x9d means a fluid having a nitrogen concentration equal to or greater than 99 mole percent.